1. Field of the Invention
This invention relates to the mixed acid process for nitrating aromatic hydrocarbons. It is more particularly directed to a procedure in that process for removing water from the reaction mass.
2. Description of the Prior Art
The nitration of aromatic hydrocarbons, especially benzene, is of great commercial importance. Although many nitration processes are known, the one most widely used is the continuous mixed acid nitration.
In that process, an aromatic hydrocarbon compound and nitric acid are brought together in a medium of concentrated sulfuric acid. The sulfuric acid acts on the nitric acid to form nitronium ion (NO.sub.2.sup.+), which is the actual nitrating agent.
The rate of reaction in the process is governed by the concentration of nitronium ions in the medium, higher concentrations of course giving faster nitration. The concentration of nitronium ions, in turn, is governed by the concentration of sulfuric acid in the medium. As the nitration proceeds, water is continuously brought into the medium with the nitric acid feed, and as a reaction product. This water dilutes the medium and lowers its sulfuric acid content to the point where the nitration reaction slows, or even stops.
It is therefore necessary to continuously remove water from the reaction medium to keep the sulfuric acid content at its optimum of about 70-75%, by weight, and thus maintain the reaction at maximum efficiency. The most common method of doing this is to continuously run the medium through a concentrator.
From a practical standpoint, this method of water removal is generally satisfactory, but recent emphasis on pollution control and energy conservation has made it less desirable because the method generates sulfuric acid mist which must be dealt with, and because large amounts of energy are required. Besides this, the concentrators are expensive because they must be made of materials strong enough to withstand constant exposure to the strong acid.